1. Field of the Invention
This invention relates to a radioactivity measurement method, especially to a method of determining radioactivity of low energy beta rays.
2. Description of the Prior Art
For thirty years nuclides whose radioactivities can be measured with a liquid scintillation counter were limited practically to tritium and carbon-14. External standard and sample channel ratio methods have been used to determine radioactivities of these two nuclides with the counter. These methods need a set of quenched standards which contain the same amount of radioactivity and differ in quenching strength. Radioactivity of a sample to be measured is determined through a quenching correction curve constructed from the quenched standards. However, only the quenched standards used for tritium and carbon-14 are commercially available, other kinds being not sold owing to the short life of the radioactivity. Furthermore it is rather difficult to prepare them for oneself.
From the above point of view, only radioactivity for tritium or carbon-14 is obtainable, but other nuclides were confined to find only their counted values.
An automatic efficiency tracing technique which eliminates the foregoing inconvenience and makes it possible to measure radioactivities of various pure beta and beta-gamma emitters have been recently developed (references: H. Ishikawa, et al.: Int. J. Appl. Radia. Isot. Vol. 35, 463 (1984); U.S. Pat. No. 4,761,555).
The conventional automatic efficiency tracing technique has the following contents. FIG. 1 illustrates a spectrum 1 of a standard and a spectrum 2 of a sample to be measured in a memory of a multichannel pulse height analyzer installed inside the liquid scintillation counter. In the FIG. 1, letting channel numbers of lower and upper levels be N.sub.l . . . N.sub.m and N.sub.u, and letting counting efficiencies of the standard in each measuring region N.sub.l -N.sub.u . . . N.sub.m -N.sub.u be E.sub.l . . . E.sub.m (%), and further counted values of the sample be n.sub.l . . . n.sub.m (cpm), the relation between E and n is expressed in the following regression equation; EQU n=a.sub.l E.sub.l.sup.2 +bE.sub.l +c.sub.l ( 1)
where a.sub.1, b.sub.1 and c.sub.1 are constant.
In the FIG. 1, a region capable of practical measurement is confined to over sensitive level (it is herein supposed to be N.sub.l), and a region under the level is not measurable. However, radioactivity of the sample is found by obtaining the total area of the spectrum which lies from null to infinity. Counting rate of the sample at counting efficiency 100% in the regression equation (1) means radioactivity of the sample. Namely, if a value at the counting efficiency 100% with respect to the standard can be found, a value (i.e. radioactivity) at that point with respect to the sample is also obtainable.
The situation to find the radioactivity of the sample as above described is illustrated by a regression curve 3 of FIG. 2, in which a crossing of the regression curve with a vertical axis means the radioactivity of the sample.
The conventional automatic efficiency tracing technique is very worthy of practical use for .beta.-emitters whose energy is more than that of carbon-14. However, tritium is too low a beta energy emitter (maximum energy is 18.6 keV), and especially it was impossible to measure a quenched tritium sample with the conventional efficiency tracing technique because of narrow spectral distribution.
Since tritium is very often used and one of the most important nuclides, the above matter was a serious drawback of the conventional technique.
The conventional efficiency tracing technique requires a standard capable of giving a high counting efficiency enough to extrapolate up to a counting efficiency 100%. If the quenched tritium sample were measured using a standard which provides counting efficiency more than 90%, the spectral distribution of tritium would be very narrow compared with respect to that of the standard. Division of the quenched tritium spectrum into the conventional measuring regions unenables an efficient tracing technique to be carried out on account of very few measured values. Each measured value in more than at least five measuring regions is required based on measurement accuracy.
Otherwise, if neighboring lower levels closely approach so as to obtain a sufficient number of measuring regions, counting efficiencies of the conventional standard scarcely changes to show a regression curve 4 in FIG. 2. Such a short regression curve 4 never derives an accurate value, even though an extrapolated value up to the counting efficiency 100% is sought by means of equation (1).